My take is this. Kubiak wants a playmaking WR downfield. He does know however that Martin and Jean have performed well in preseason only up to this point. I think he starts Walter simply because he has game experience. If by week 10 or so Jean is still performing better than Walter he will take Walters spot. Just my take.

Note: If only Martin plays better he still will not supplant Walter because he is not suited for playing outside.